The formation of various integrated circuit (IC) structures on a wafer often relies on lithographic processes, sometimes referred to as photolithography, or simply lithography. As is well known, lithographic processes can be used to transfer a pattern of a photomask (also referred to herein as a mask or a reticle) to a wafer.
For instance, patterns can be formed from a photo resist layer disposed on the wafer by passing light energy through a photomask having an arrangement to image the desired pattern onto the photo resist layer. As a result, the pattern is transferred to the photo resist layer. In areas where the photo resist is sufficiently exposed and after a development cycle, the photo resist material can become soluble such that it can be removed to selectively expose an underlying layer (e.g., a semiconductor layer, a metal or metal containing layer, a dielectric layer, a hard mask layer, etc.). Portions of the photo resist layer not exposed to a threshold amount of light energy will not be removed and serve to protect the underlying layer during further processing of the wafer (e.g., etching exposed portions of the underlying layer, implanting ions into the wafer, etc.). Thereafter, the remaining portions of the photo resist layer can be removed.
There is a pervasive trend in the art of IC fabrication to increase the density with which various structures are arranged. For example, feature size, line width, and the separation between features and lines are becoming increasingly smaller. For example, nodes with a critical dimension of about 45 nanometers (nm) to about 65 nm have been proposed.
In these sub-micron processes, yield is affected by factors such as optical proximity effects and photo resist processing. Characteristics of the equipment used to image the desired pattern onto the photo resist can also play a large role in image fidelity and the quality of the resulting integrated circuit. Therefore, it may desirable to characterize or otherwise quantify the performance of lithography equipment. Currently, resist images are used to quantify characteristics of lithography equipment. However, this technique is relatively imprecise and cannot be effectively used to characterize an individual subassembly of a lithography system, such as an illuminator for use in conjunction with a stepper or a scanner.
Accordingly, there exists a need in the art for an improved method and apparatus for characterizing the behavior of certain features of the lithographic equipment. There is a further need to compensate for the behavior variations of such equipment to improve integrated circuit manufacture.